1. Field of the Invention
This invention relates to a photoresist coating machine, and more particularly to improving a dispensing system used in a photoresist coating machine.
2. Description of Related Art
Photolithography plays an essential role in semiconductor fabrication. All semiconductor devices need several photolithography process to transfer desired patterns so as to form the devices as designed. A semiconductor device usually includes, for example, a transistor structure with proper doping regions, a capacitor, and an interconnecting structure for connection between each component. All these needs several different photolithography processes. A more complex structure accordingly needs more photolithography processes.
The detailed photolithography technology is usually complicated but its theory is straightforwardly simple. Generally, a photoresist layer is coated on a device wafer surface, on which a device is fabricated. The photoresist layer then is exposed by a light source through a photomask, which carries a pattern to be transferred onto the device wafer surface. The photoresist layer includes photo-sensitive material that can be exposed to light to selectively change its material property. After development, the remaining photoresist layer on the device wafer surface forms the desired pattern. The desired pattern therefore is transferred to the device wafer. Up to this stage, the photolithography process is done. A subsequent process, such as etching or doping, are performed to form one sub-structure of the device, which usually includes several different sub-structures. So, a more complex structure accordingly needs more photolithography processes.
Currently, the photoresist layer can be formed by a spin coating process so as to obtain its thickness uniformity and adhesion without defects. FIG. 1 is a side view of a conventional spinner used for spin coating. A wafer 12 is held by a spinner 10. The spinner 10 fixes the wafer 12 by sucking it with a vacuum force, which is created through a rotating axle of the spinner 10. As the wafer 10 is rotated by the spinner 12, a liquid photoresist 14 sprayed on the wafer 12 is outwardly distributed due to centrifugal force. The liquid photoresist 14 contains volatile organic solution. After volatilization, a uniform photoresist layer 16 with strong adhesion is formed over the wafer 10.
FIG. 2 is schematic drawing of a conventional photoresist dispensing system included in a photoresist coating machine. In FIG. 2, the photoresist coating machine is, for example, a D-SPIN type. A conventional photoresist dispensing system includes a transporting part and a control part. The transporting part includes a container 21 that contains a liquid photoresist 20. A pump 23 is used to pump the liquid photoresist 20 from the container 21 through a transporting duct 90. The pump 23 transports the liquid photoresist 20 to a filter 24 and then to an orifice valve 25, which determines whether the liquid photoresist 20 is to be sent to a wafer 22 or not. If the liquid photoresist 20 is to be sent to the wafer 22, the liquid photoresist passes a sucking-back valve 26 and is transported along the duct 90.
The control part includes a solenoid valve 27 to control the actions of the orifice valve 25 and the sucking-back valve 26 by switching between two modes. One is a starting mode that the dispensing system is switched from a resting status to a working status. The other one is a stopping mode that the dispensing system is switched from a working status to a resting status. The control part uses air-flow to achieve its purpose of control. The air flows along a route 91. The solenoid valve 27 includes an air-in end AIR and an air-out end EXH. Air can be injected from the air-in end AIR and exhausted from the air-out end EXH. The solenoid valve 27 uses the EXH and the AIR to switch system between the starting mode and the stopping mode. The sucking-back valve 26 is controlled by the solenoid valve 27 through air-flow to produce or release a sucking force, which suck back the liquid photoresist 20. The orifice valve 25 is controlled by the solenoid valve 27 through air-flow to determine whether the liquid photoresist 20 is transported to the wafer 22 or not.
Between the solenoid valve 27 and the orifice valve 25, there are two speed controllers SC1, SC2. The speed controller SC1 is coupled between the AIR of the solenoid valve 27 and the orifice valve 25 at a first input/output (I/O) end 30. The speed controller SC2 is coupled between the EXH of the solenoid valve 27 and the orifice valve 25 at a second I/O end 33. Each speed controller used in the photoresist dispensing system is identical and is used to control a switching speed of action. The speed controllers SC1, SC2 therefore control the switching speed of action for the orifice valve 25.
A sucking-back speed unit 41 is coupled between the EXH of the solenoid valve 27 and the sucking-back valve 26 at a third I/O end 40. The sucking-back speed unit 41 includes two speed controllers SC5, SC6.
Moreover, the solenoid valve 27 also control the pump 23 to start pumping the liquid photoresist 20 to the orifice valve 25 through the one-way valve 24. A speed controller SC3 is coupled between the AIR of the solenoid valve 27 and the pump 23 at a first air control end 36 and a speed controller SC4 is coupled between the EXH of the solenoid valve 27 and the pump 23 at a second air control end 39. The combined result of the first air control end 36 and the second air control end 39 controls the pump 23 for whether pumping the liquid photoresist 20 or not.
In the above coupling manner, the solenoid valve 27 can switch, or control, the pump 23, the orifice valve 25 and the sucking-back valve 26 for photoresist dispensing. Ideally, when the dispensing system is operated at the stopping mode, the orifice valve 25 and the pump 23 are switched off so that the liquid photoresist 20 is not dispensed. Then, the sucking-back valve 26 produces a sucking force on the liquid photoresist 20 so that the liquid photoresist 20 at a duct end 42 is sucked back a little as shown in a magnified drawing. The end surface 43 of the liquid photoresist 20 is concave. The liquid photoresist 20 is avoided to drop a little onto the wafer 20, resulting in a deterioration of the quality of a formed photoresist layer (not shown). Practically, it is difficult for the conventional coupling manner to achieve the ideal operation result.
Similarly, when the dispensing system is switched to the starting mode, the sucking-back valve 26 should ideally release its sucking force before the pump 23 and the orifice valve 25 start operation. Otherwise undesired drops of the liquid photoresist 20 may drop onto the wafer 22 before photoresist coating operation starts. This also deteriorates the quality of the photoresist layer, which is to be formed. Practically, it is difficult for the conventional coupling manner to achieve the ideal operation result.
In summaries, the conventional photoresist dispensing system used to coat a photoresist layer on a wafer has a problem to properly control the action time order of the pump 23, the orifice valve 25 and the sucking-back valve 26. This may cause a few undesired drops of the liquid photoresist 20 to drop onto the wafer 34 and to deteriorate the quality of a photoresist layer to be formed.